Using Markdown++ you can include the contents of one markdown file in another, similar to the following:

<!--include:../Topics/MyTopic.md-->

In the above statement, a filename from the parent folder's Topics folder is being included in markdown file.

To include markdown files in other folders, be sure to use a path that is relative to the file that contains the include statement. Absolute paths will work as well, but might not be as portable if you move your content from one location to another.